This invention is related generally to the quantification of properties in high pressure die cast (HPDC) aluminum alloys, and in particular to an improved quantitative metallographic methodology to accurately measure skin layer thickness in such cast components.
HPDC (also referred to as die casting) is being used extensively in the production of lightweight aluminum alloy components in general, and particularly for automotive components, such as engine blocks and transmission cases, as well as pistons or suspension parts. Low costs for large-scale production, close dimensional tolerances (near-net-shape) and smooth surface finishes are all positive attributes that make HPDC so attractive. Unlike alloys (such as 319 or 356) that are not typically used in HPDC, certain aluminum alloys, such as 380, 383, 390 or the like, are particularly well-suited to HPDC for their cost, strength, fluidity and generally good corrosion resistance qualities.
One disadvantage of the conventional HPDC process is that the parts are not amenable to heat treatment due to the presence of porosity in the casting that arises out of the dynamics peculiar to the HPDC process. As such, HPDC-produced aluminum parts are generally considered as having an outer skin region surrounding an inner region. In such structure, the region typically associated with the skin exhibits a relatively defect-free, dense microstructure, and has better mechanical properties than the region associated with the internal areas, where the voids, porosity and related defects are present. These defects are generally attributable to various factors one of which is shrinkage of the alloy from a low density liquid metal to a high density solid casting during solidification. Another contributing factor is the formation of gases, such as hydrogen or vapors from the decomposition of die wall lubricants, while still another factor is any entrapped air that occurs due to the rapidity with which the die is filled with the molten metal.
Thus, HPDC presents unique design challenges. Despite the location-specific nature of the mechanical properties mentioned above, conventional design approaches assume the presence of uniform microstructure and properties across the entirety of the cast component; much of this is due to the difficulty with which to accurately determine skin layer thickness. Such simplifying assumptions can in turn lead to unrealistic predictions of component structural properties through finite element analysis (FEA) or a related quantitative analysis tool. Inaccurate predictions may be particularly problematic in component failure analyses, as this can lead to either expensive warranty work or inefficient overdesign of the component; in either event, such inaccuracies impact the ability of the component designer to take full advantage of HPDC materials and processes.